Environmental demands has forced our industry to develop improved cooking and bleaching methods. One recent breakthrough within the field of cooking is ITC.TM., which was developed in 1992-1993. ITC.TM. is described in WO-9411566, which shows that very good results concerning the pulp quality may be achieved. ITC.TM. is mainly based on using almost the same temperature (relatively low temperature compared to the prior art) in all cooking zones in combination with moderate alkaline levels. The ITC.TM.-concept does not merely relate to the equalization of temperatures between different cooking zones, but a considerable contribution of the ITC.TM.-concept relates to enabling an equalized alkaline profile also in the lower part of the counter-current cooking zone.
Moreover, it is known that impregnation with the aid of black liquor can improve the strength properties of the fibers in the pulp produced. The aim of the impregnation is, in the first place, to thoroughly soak each chip so that it becomes susceptible, by penetration and diffusion, to the active cooking chemicals which, in the context of sulphate cellulose, principally consist of sodium hydroxide and sodium sulphide.
If, as is customary according to the prior art, a large proportion of the white liquor is supplied in connection with the impregnation, there will exist no distinct border between impregnation and cooking. This leads to difficulties in optimizing the conditions in the transfer zone between impregnation and cooking.
Now it has been found that surprisingly good results can be achieved when:
1. Keeping a low temperature but a high alkali content in the beginning of a concurrent cooking zone of the digester;
2. Withdrawing a substantial part of a highly alkaline spent liquor that has passed through at least the concurrent cooking zone; and
3. Supplying a substantial portion of the withdrawn spent liquor that has a relatively high amount of rest-alkali, to a point that is adjacent the beginning of an impregnation zone.
This leads to a reduced H-factor demand, reduced consumption of cooking chemicals and better heat-economy. Additionally, the novel method leads to production of pulp that has a high quality and a very good bleachability, which means that bleach chemicals and methods can be chosen with a wider variety than before for reaching the desired quality targets (brightness, yield, tear-strength, viscosity, etc.) of the finally bleached pulp.
Furthermore, we have found that these good results can also be achieved when moving in a direction opposite the general understanding of the ITC.TM.-teaching, in connection with digesters having a counter-current cooking zone. Instead of trying to maintain almost the same temperature levels in the different cooking zones, we have found that when using a digester that has both a concurrent and a counter-current cooking zone, big advantages may be gained if the following basic steps are used:
1. Keeping a low temperature but a high alkali content in the concurrent zone of the digester;
2. Keeping a higher temperature but a lower alkali content in the counter-current zone;
3. Withdrawing a substantial part of the highly alkaline spent liquor that has passed through at least one digesting zone; and
4. Preferably supplying almost all of the withdrawn spent liquor, that has a relatively high amount rest-alkali, to a position that is adjacent the beginning of the impregnation zone.
Also, in connection with digesters of the one-vessel type (without a separate impregnation vessel), surprisingly good results are achieved when the basic principles of the invention are used. The good results also apply to digesters having no counter-current zone and to overloaded digesters that cannot be provided with a sufficient supply of wash liquor to enable a sufficient up-flow for counter-current cooking.
Moreover, preliminary results indicate that the preferred manner of using the invention may be somewhat modified also in other respects but still achieving very good result, e.g., by excluding the counter-current cooking zone. Additionally, expensive equipment might be eliminated, e.g., strainers in the impregnation vessel, hanging central pipes, etc., making installations much easier and considerably less expensive.